[Illustration: Fig. 119.--Electric organ of the skate. The left-hand drawing (i) represents the entire organ (natural size) of a full-grown _r. radiata_. This is a small skate, which rarely exceeds 50 centms. in length; but in the large _r. batis_, the organ may exceed two feet in length. The other drawings represent single muscle-fibres in successive stages of transition. In the first of the series (ii) the motor plate, and the nerves connected with it, have already been considerably enlarged. In the other three specimens, the fibre becomes more and more club-like, and eventually cup-like. These changes of shape are expressive of great changes of structure, as may be seen in the last of the series (v), where the shallow cup is seen in partial section. The electric plate lines the concavity of the cup, and is richly supplied with nerves (only a few of which are represented in the last drawing); the thick walls of the cup are composed of muscular fibres, the striation of which is distinctly visible.] [Illustration: Fig. 120.--Electric cells of _raia radiata_. The drawing on the left represents one of the clubs magnified, as in the preceding wood-cut. The drawing on the right represents a number of these clubs, less highly magnified, _in situ_.] Again, it cannot be answered that the principle of correlation may be drawn upon in mitigation of the difficulty. The structure of the electric organ is far too elaborate, far too specialized, and far too obviously directed to a particular end, to admit of our conceivably supposing it due to any accidental correlation with structural changes going on elsewhere. Even as regards the initial changes of muscle-elements into electrical-elements, I do not think the principle of correlation can be reasonably adduced by way of explanation; for, as shown in the illustrations, even this initial change is most extraordinarily peculiar, elaborate, and specialized. But, be this as it may, I am perfectly certain that the principle of correlation cannot possibly be adduced to explain the subsequent _association of these electrical elements into an electric battery_, actuated by a special nervous mechanism of enormous size and elaboration--unless of course, the progress of such a structure were assumed to have been throughout of some utility. Under this supposition, however, the principle of correlation would be forsaken in favour of that of natural selection; and we should again be in the presence of the same difficulty as that with which we started.